Progress Reporting in PowerProject.

Have just converted from P3 to PowerProject v.8 and am struggling to enter and present progress. PowerProject seems to have a direct link between Actual Start / End and Percentage Complate. I dont understand the reasoning behind this, as clearly an activity can take longer or shorter than originally intended, and just because a 10 day activity STARTED 5 days before the report, does not necessarily mean 50% of the work is completed. However, this is the assumption the software makes.

sorry to be vague, but I’m hoping someone out there understands me !